There has recently been a growing demand for autonomic decentralization of a machining system in an entirety of a machine system including robots and machine tools wherein the machining system is divided into independent functional units having control functions and machining functions of their own, i.e., autonomous units, wherein these units, maintaining their independence, carry out a coordinated machine work by effecting communications between a static functional unit (hereinafter referred to as a static unit) and movable functional units as well as mutually between the movable functional units for exchanging commands and information.
Dividing the machining system into a plurality of functional units in this way enables the optimum combinations of a plurality of functional units to fit each particular working object as the occasion demands, and consequently, offers the advantage that a single machine plant may serve for carrying out a wide variety of functions.
However, dividing the processing system into functional units is by no means obvious. For example, determining what extent of the apparatuses within the machining system should be consolidated as one unit presents one technical problem. In addition, it is to be desired that the attachment and removal of each functional unit be easy, and moreover, that an electric power transfer system and a communication system be automatically established between a movable functional unit and the static unit upon attachment of the movable functional unit to the static unit. This is particularly essential in the field of machine tools for realizing complete automatization of operation of jigs and for electrically controlling such operations as positioning, centering, and clamping a workpiece on a pallet as it moves from a setup process to a work process.
In addition, even in the case that a functional unit, for example a servomotor, is not physically separable from the static unit, a way of controlling the servomotor driven on a rotating body making multiple rotations is also becoming desirable. This applies to such cases as, for example, the rectilinear drive of a machine post mounted at the tip of the main shaft of a machine tool by an electric motor, or the electrical powering of a lathe head chucking section, or to a case in which the main shaft of an electric motor is mounted on a rotating index table. In such a case, in order that the control signals and electric power to drive the electric motor is supplied from a static unit, it is necessary that the electric power supply system and communication system always operate stably for any rotations of the rotating body.
A general survey will next be presented of the prior art relevant to the present invention from the viewpoint of the above-described current state of mechanical engineering.
FIG. 1 is a block diagram showing the basic structure of an electric motor control of the prior art.
A power source 11 inputs electric power of commercial frequency and supplies main power supply S12 and control power supply S13 to controller 12. The controller 12, driven by control power supply S13, is composed of position amplifier 12.sub.1, speed amplifier 12.sub.2, differentiator 12.sub.3, current amplifier 12.sub.4, and power switch 12.sub.5, thereby modulating and supplying the main power supply S12 to the servomotor 13 in response to a position command S11 fed from the upstream system. The detector 14 detects the position of the servomotor 13 and feeds back a position signal S15 to the position amplifier 12.sub.1 (position loop). The position amplifier 12.sub.1 generates a speed command from position command S11 and position signal S15. The differentiator 12.sub.3 differentiates position signal S15 and generates a speed signal. The speed amplifier 12.sub.2 inputs the speed signal and speed command and outputs a torque command (speed loop). The current amplifier 12.sub.4 compares the torque command and current signal (current detector value) S14 and modulates the current to be supplied to the servomotor 13 by controlling the power switch 12.sub.5. In this way, control of the prior art of a servomotor is carried out with a servocontroller system including a power source, a position detector and a servocontroller all being fixed based on the premise that any of the constituent parts will not be removed.
In the field of machine tool working, work is carried out for example, by controlling the positioning of a tool post 24 at the end of a main shaft 21 (facer machining center) as shown in FIG. 2, or by chucking a workpiece 34 through chucking jaws 33 driven by a chucking motor 32 at the shaft end of a main motor 31 or spindle unit as shown in FIG. 3 through signal communication with the rotation shaft and through additional power supply other than the rotation power to the rotating shaft. In such cases, however, because the supply of electric power and signals could not be easily achieved in the prior art, methods have been used such as arranging, within a hollow shaft of the main motor 41 or spindle unit, a coaxial shaft 43 for transmitting power in the form of mechanical power, as shown in FIG. 4, but due to problems relating to machining accuracy and long-term reliability, it has been extremely difficult to put this approach into actual use at a low cost. FIG. 4 shows a case in which the mechanical power is used to drive bevel gears 44.sub.1, 44.sub.2 to move a traveling pedestal.
In machine tool working, there has also been great demand for controlling an actuator provided at an end of a main shaft, and otherwise for sending information to a workpiece, jig, or tool at the end of a main shaft, or monitoring the conditions of these components by means of detectors. For example, in a chucking device attached at an end of a spindle head driven with a hydraulic cylinder, because the effective chucking pressure decreases due to the centrifugal force acting on the workpiece as the rotational speed of the spindle increases, it is desirable to effect on-line control through feedback of chucking pressure. Even when actual control of the chucking pressure cannot be realized, there remains a demand for on-line monitoring of the chucking pressure.
In-process monitoring of the state of a tool attached at the end of a spindle head during machining, prediction of breakage of a tool or confirmation of breakage is an essential item for effecting continuous 24-hour processing in FMC. For this reason, it is desirable to have sensor information (for example, information on tool tip temperature, vibration, acoustic emission, etc.) sensed at the main shaft end and returned during machining to an NC control device in real time. It is furthermore necessary to have measured position information on the location ahead of the spindle head (for example, the gap between tool and workpiece), and more basically, feedback of sequence signals such as limit switch signals in an ATC collect chuck.
As explained above, despite the strong demand for obtaining on-line information beyond the main shaft, transmission of the information by wiring cannot be used, because this involves a difficulty of wiring from a part rotating at high speed to a static part. Further, in order to obtain this information reliably, the detectors must in nearly all cases be mounted at the end of the main shaft, and consequently, the detectors must be supplied with electric power from the outside. Mounting batteries at the end of the main shaft to supply power is conceivable but usually not practical due to a large increase in weight of the portion that rotates at high speed as well as to the difficulty of exchanging batteries. Directly coupling a rotary electric generator to the main shaft to obtain power through rotation of the main shaft is also conceivable, but this course would not provide sufficient power when the shaft is at rest or rotating at a low speed. As a result, the necessity remains for some method of transmitting electric power for the detectors from the static part to the end of the main shaft, and conversely, transmitting detector information from the end of the main shaft to the static part, by way of the high-speed rotating part and independently of the rotating state of the main shaft.
As a method of the prior art, there are examples in which power supply and signal transmission are carried out by arranging slip rings coaxially with the main shaft, but this method has proved impractical in such a case as the main shaft rotates at high speeds of over several thousand rpm, because there is a tendency for problems such as noise generation caused by contact abrasion and poor contact.
In multi-articulated robots and SCARA robots, power supply and signal communication for every output shaft of servomotors have been achieved with a large number of wires, but problems are encountered in that the range of movement of the robot arm is restricted by turn-aside of the wiring and long-term repeated operations lead to fatigue and breakage of the wiring.
Regarding multi-articulated robots, a solution to the above-described turn-aside problem has been proposed in Japanese Patent Laid-open 93-13796. In this multi-articulated robot, a first arm is driven by a direct-drive motor installed in a static shaft. A second arm and a tool shaft are driven by way of pulleys supported by the static shaft, the rotation shaft of the second arm, the tool shaft and rotation transmission means (time belt) linking the pulleys. As to the wiring, a first slip ring is provided around the outside of the direct drive motor for driving the first arm, a third slip ring is provided around the outside of the tool shaft at the end of the second arm, and wiring within the base is connected by way of the first slip ring to the third slip ring through the hollow rotation shaft at the end of the first arm, and further, is connected to the hand through the hollow tool shaft. In this way, the first arm, second arm, and wrist do not interfere with each other and rotation greater than 360.degree. is possible without tangling or break of the wire. However, in this multi-articulated robot, the slip ring is used for the transfer of electric power and signals to the tool shaft.
In addition to the use in multi-articulated robots as described above, contact slip rings have been used for supplying power and communicating signals to multiple-rotation bodies, but here, improvement of reliability is limited by problems of stability and electrode wear during high-speed rotation, and when assembled in a machine, exchange operations are difficult. Furthermore, the adoption of this method of electrode contact is rendered essentially impossible due to problems of maintaining reliable electrical contact when exposed to the metal chips and cutting oil mist present in the working ambience of working machinery.